The present invention relates to a novel curable resin composition comprising an organopolysiloxane and an epoxy resin and capable of giving a cured article of the composition which is excellent in various properties such as mechanical strengths, moisture resistance, heat resistance and mold releasability as well as transparency to be useful in a variety of application fields.
As is known, epoxy resin-based curable compositions in general give cured articles having excellent properties such as mechanical strengths, adhesiveness and the like as compared with other thermally curable resins so that, along with the advantage of the relatively low costs, they are practically used in large quantities in various application fields including coating compositions and adhesives, electrical industries, architectural and civil engineerings and so on. As the application field of epoxy resins is expanded year by year, the requirements-for the epoxy resins are also under continuous upgrading toward higher heat resistance, weatherability, moisture resistance and flexibility of the cured articles thereof as well as workability in the molding works thereof in relation to the mold releasability.
It is also known that, on the other hand, silicone resins are widely used as a material of electric and electronic instruments, transportation machines, precision machines and the like by virtue of their excellent heat resistance, weatherability, moisture resistance and electrical properties. Silicone resins, however, are not always quite satisfactory in these applications because the mechanical strength of the cured articles of a silicone resin is relatively low as compared with organic resins. This deficiency of silicone resins of course can be remedied to some extent by compounding the silicone resin with a reinforcing inorganic filler such as finely divided silica fillers but the reinforcing effect given by such an inorganic filler is not so high in silicone resins as in silicone rubbers and compounding of a silicone resin with a large amount of an inorganic filler necessarily causes difficult problems in respect of the workability in the molding works due to the decrease in the flowability of the silicone resin composition at the molding temperature.
It would be a due idea that the above mentioned defects of silicone resins in general could be remedied by blending the silicone resin with a suitable organic resin capable of compensating the deficiency of the silicone resin. Within the approach in this way, various kinds of organic resins have been proposed and tested heretofore as a blending component in silicone resins of which epoxy resins are a class of promising ones. Epoxy resins as a compounding resin with silicones, however, have a problem in respect of their relatively low compatibility or miscibility with silicone resins so that uniform resin blends can hardly be obtained from a silicone resin and an epoxy resin.
An alternative method is proposed in which the organopolysiloxane molecules of the silicone resin are modified with epoxy groups so as to improve the compatibility of the silicone resin and an epoxy resin. This approach is not practical because of the remarkable decrease in the mechanical strength of the epoxy resin due to the silicone resin if not to mention the expensiveness of such an epoxy-modified silicone resin as a consequence of the specific modification reaction.